1. Field of the Invention
The present invention relates to injection molding systems and relates in particular to the injection of metered amounts of melt and to the injection of melt at high pressure.
2. Related Art
In many applications it is desirable to mold plastic parts with the least amount of plastic necessary to perform the desired function of the finished part without premature failure. Therefore, as resins are made increasingly stronger, part wall thickness can correspondingly be made thinner and more molded parts can be made with the same amount of melt. In addition, since thinner parts are quicker to cool, set and eject, parts with thinner walls can be made at a faster cycle time than parts with thicker walls, which increases maximum machinery output rates.
Thinner parts generally require higher injection pressures than thicker parts of similar size and shape. Therefore, machinery injection units capable of creating increasingly higher injection force are required to fill mold cavities for increasingly thin-walled parts. Prior designs attempting to meet this need have utilized high pressure injection units coupled with hot runner manifold systems capable of withstanding high pressures. These high pressure injection units and manifold systems are often more expensive and more difficult to maintain because higher quality materials capable of withstanding high pressures must be used. These systems also suffer from the fact that a significant amount of pressure is lost as the melt passes through the manifold and the nozzle, which makes achieving desired pressures within the mold cavity more challenging still.
In many applications it is also desirable to reliably produce molded parts with statistically consistent part characteristics. In many instances customers require stringent and repeatable molding processes to be verified with sensors, instrumentation and/or fixed and documented molding parameters. One area of particular concern is part weight, which is perceived as an indication of complete part filling and consistency of part quality and/or uniformity.
In many prior designs, this is accomplished by precision design and manufacturing of hydraulically balanced melt channel layouts, carefully thermally balanced heat distribution of the manifold and nozzles, use of valve gated cavity filling orifices in the manifold, and valve pin position sensors to confirm the opening and closing of each cavity position during the injection cycle.